Dutch Patent Application No. 6601148 discloses a process for preparing adipic acid by oxidizing 1,2-dioxycyclohexane in a nitric acid aqueous solution containing a water-soluble vanadium salt. It discloses that the advantage of the process, over the currently more common process of oxidizing cyclohexanol and/or cyclohexanone with nitric acid, is that the proportion of nitric acid consumed in the form of nitrogen and nitrous oxide is reduced. However, in Example 1 in which cyclohexene oxide is used as a starting material, and nitric acid having a concentration of 70% is used as the oxidizing agent, the yield of adipic acid attained is 92%, and 0.12 mol of nitric acid is consumed in the form of nitrous oxide and nitrogen per mol of the organic starting material. In Example 4 in which 1,2-dihydroxycyclohexane is used as a starting material, and nitric acid having a concentration of 70% is used as the oxidizing agent, the yield of adipic acid is 94%, and 0.16 mol of nitric acid is consumed in the form of nitrous oxide and nitrogen per mol of the organic starting material. Therefore, while catalysis by water-soluble vanadium salts gives some improvement, the process of the Dutch application is still unsatisfactory.
while 1,2-dihydroxycyclohexane can be obtained through various processes, hydration of cyclohexene oxide derived from cyclohexene is one of the industrially more economical processes. A method of hydration of an epoxy compound is described, e.g., in Morison Boyd Yuki Kagaku Dai-4-han (Morison Boyd Organic Chemistry 4th edition), translated by Nakanishi et al., pp. 720-728, Tokyo Kagaku Dojin. Applicants have found that the hydration of cyclohexene oxide is accompanied by the formation of by-products consisting mainly of oligomers of cyclohexene oxide represented by formula (I): ##STR2## wherein n represents a number of from 1 to 5 in number average. Applicants have observed that water for hydration must be used in large excess over a theoretical amount in order to minimize the by-production of oligomers (I).